Conventional DC power supplies need different rated output powers to meet customer's demands. However, power supplies with different rated output powers require different models, which is not cost-effective. Referring to FIG. 1, the industry often uses the same casing 1 and a power module 2 with a single output power. As shown in FIG. 2, the bottom of the power module 2 is provided with a base 3. A fan 4 is disposed at the front end of the power module 2. An upright board 5 is longitudinally disposed on one side of the power module 2. A circuit board 6 is fixed on the upright board 5. Depending on the rated output power required by the power supply, the corresponding power modules 2 are installed in the casing 1. As shown in FIG. 3, three power modules 2 are installed in the casing 1. The circuit boards 3 of the power modules 2 are connected in series through control wires 7 each having an equal length, and then connected to a main circuit board 8. The power modules 2 connected in series supply the required rated output power. For example, when the output power of the power module 2 is 5 kw, if the DC power supply is equipped with three power modules 2, the total rated output power is 15 kw. When the rated output power of the DC power supply only needs 5 kw or 10 kw, one power module 2 or two power modules 2 connected in series are installed in the casing 1. Since the internal space of the casing 1 is not fully occupied by the power modules 2, in order to maintain the center of gravity of the casing 1, it is required to adjust the position of the power module 2 installed in the casing 1. Referring to FIG. 4, when only two power modules 2 are installed in the casing 1, the power modules 2 are installed on the left and right sides of the casing 1 to maintain the center of gravity of the casing 1. An empty space is formed between the two power modules 2. The original control wires 7 cannot be connected to each other through the circuit boards 6 of the power modules 2, so that the power modules 2 need to be connected by control wires 9 of different lengths. This causes troubles in wiring, degrades the output quality of the power supply, and brings inconvenience in maintenance. In addition, since one side of one power module 2 is unobstructed due to the middle empty space of the casing 1, the cooling wind sent by the cooling fan of the power module 2 dissipates outward, which affects the heat dissipation effect of the power module Accordingly, the inventor of the present invention has devoted himself based on his many years of practical experiences to solve these problems.